Digital subscriber line (DSL) technologies can provide relatively large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent twisted-pair phone lines, for example in a same or nearby bundle of lines. Crosstalk limits the performance of some DSL technologies, such as asymmetric DSL 2 (ADSL2) and very high bit rate DSL 2 (VDSL2). For example, significant levels of crosstalk can occur because of the relatively high frequencies used in VDSL2. Crosstalk can occur in mixed deployment scenarios, where cabinet and exchange deployed lines operate within relatively short proximity or in the same binder. As such, relatively high power transmissions on shorter lines or loops (e.g. deployed by the cabinet) can result in relatively high levels of “far-end” crosstalk (FEXT) on longer loops (e.g. deployed by the exchange). To reduce the crosstalk for upstream transmissions, e.g. to the cabinet and/or exchange, a spectrum management technique referred to as upstream power back-off (UPBO) can be used. The UPBO technique reduces the upstream transmit power spectral density (PSD) on shorter loops. Hence, the upstream transmission PSDs and the FEXT can be equalized, e.g. become weakly dependent on loop characteristics, such as loop length.
The crosstalk can be highly non-stationary and vary dramatically as lines within the binder are activated and deactivated. Such rapidly varying noise environment causes frequent re-initializations of the lines, loss of service, and low customer satisfaction. One method for increasing the stability of digital subscriber lines is using virtual noise, a technique that limits the maximum bit-loading allowed on each tone. Virtual noise is used to improve DSL stability by ensuring operation of a line at a bit-loading level that can be sustained in worse noise conditions. The performance of a DSL is substantially dependent on the virtual noise configuration for the line. For example, if the virtual noise is set too low, the line becomes unstable. Alternatively, if the virtual noise is set too high, the line's data-rate is unnecessarily reduced. Using both the virtual noise technique and the UPBO technique to reduce crosstalk in the lines is desirable but has complications due to compatibility issues between the two techniques.